The present invention relates to an optical element provided with a structure having a reflection suppressing effect (anti-reflection function), and to an optical apparatus including the optical element.
In general, on at least one surface of an optical element formed of a transparent member, a thin film having an anti-reflection function is formed by using a film forming method typified by vapor deposition and sputtering. However, such a film forming method limits materials that can be used for film formation, which makes it difficult to obtain a thin film having an arbitrary refractive index.
Japanese Examined patent publication No. 61-51283 discloses a method for virtually obtaining a thin film having an intermediate refractive index by selectively introducing a thin film having a high refractive index and a thin film having a low refractive index and by appropriately setting thicknesses of the introduced thin films.
Further, another method forms an anti-reflection structure constituted by plural structure portions (protrusions) each smaller than a use wavelength (that is, a wavelength of light entering the optical element) on at least one surface of an optical element. The most famous anti-reflection structure is a moth-eye structure. A surface of the moth-eye structure provides a very low reflectance due to a microstructure unique to the moth-eye structure.
In a microstructure whose each structure portion is smaller than the use wavelength, light of the use wavelength cannot recognize the microstructure and therefore behaves as if entering a uniform medium. The microstructure has a refractive index according to a volume ratio of a material forming the microstructure. The use of this can realize a microstructure having a low refractive index that cannot be obtained by using normal materials. The use of such a low refractive index microstructure can achieve a higher performance anti-reflection function.
Japanese Patent Laid-Open No. 2005-62674 discloses an anti-reflection structure formed by the above-described microstructure in which protrusions have a tapered shape toward a surface side (light entrance side). In such an anti-reflection structure, a refractive index gradually reduces from a base member side of an optical element toward the surface side thereof.
Japanese Patent Laid-Open No. 2003-240904 discloses a microstructure constituted by plural protrusions each having a shape in which, in comparison of a most convex portion and a most concave portion of the protrusion, the most convex portion is sharper than the most concave portion. Such a shape of the protrusion makes change in refractive index at a superficial surface part or a boundary part between the microstructure and the base member more gently, which reduces the reflectance of the microstructure.
However, the film virtually having the intermediate refractive index disclosed in Japanese Examined patent publication No. 61-51283 is inferior in a broadband characteristic since the film is formed by using a high refractive index material.
Japanese Patent Laid-Open No. 2005-62674 does not disclose an optimal refractive index structure though it discloses a microstructure formed so as to be tapered in order to gradually change the refractive index.
Further, the microstructure disclosed in Japanese Patent Laid-Open No. 2003-240904 is focused only on the change of the refractive index at a boundary surface, which generates part where the refractive index greatly changes, and therefore a good broadband characteristic cannot be obtained.
Thus, the film disclosed in Japanese Examined patent publication No. 61-51283 and the microstructures disclosed in Japanese Patent Laid-Open No. 2005-62674 and Japanese Patent Laid-Open No. 2003-240904 can realize an anti-reflection function under restricted conditions. However, the anti-reflection function is inferior in the broadband characteristic as well as in an incident angle characteristic.